1. Field of the Invention
This invention relates to a ceramic filter circuit for a band pass filter in an IF circuit of a radio receiver.
2. Description of the Prior Art
In most case, a ceramic filter circuit comprises a symmetrical T-shape connection of three ceramic vibrator elements, a symmetrical .pi.-shape connection of three ceramic vibrators, or a cascade connection thereof, or a single ceramic vibrator having asymmetrically divided electrodes. There are some references, as shown in FIG. 1a, wherein a relatively high resistance resistor R1 is provided in series at the input side of the ceramic vibrator CF and a resistor R2 is in shunt at the output side, thereby forming a low-pass filter together with an input stray capacitor C1 and an output stray capacitor C2, or as shown in FIG. 1b wherein a tuning coil T (which serves also as an impedance converter) is provided between a series resistor R1 and the ceramic filter element CF.
However in the circuit as shown in FIG. 1(a), a distortion occurring upon passing of the frequency modulated waves through the ceramic filter becomes larger towards the higher frequency range as shown by the curve a in FIG. 2, and differs considerably from one circuit to another due to non-uniformity of characteristics of ceramic filter elements.
With the circuit shown in FIG. 1(b), it is possible to provide a good linearity in phase characteristic of the driving impedance of the ceramic filter by a slight adjustment of the tuning coil T, but the Q factor of the tuning coil T must be set at a relatively high value to provide a compensation amount covering a characteristic fluctuation from one ceramic filter element to another, which as a result brings poor linearity in the phase characteristic of the tuning coil itself at both ends of the pass band thereby providing such a characteristic as shown by the curve b in FIG. 2. Furthermore, it is hard to secure a good safety stability against temperature since the coil is used under a condition of Q being high.
FIG. 3 shows a diagram wherein variable resistors VR1 and VR2 are connected in series at the input and output ends of the ceramic filter CF. After adjusting the variable resistors so as to provide a minimum distortion at the modulation frequency of 1kHz, the modulation frequency is sequentially varied from 40Hz to 50kHz to measure the distortion factor exhibiting a result as shown in FIG. 4. From this figure, it is observed that the distortion factor is almost flat within the range of the modulation frequency from 100Hz to 700Hz and its value is below 0.1%, but the distortion factor gradually increases as the modulation frequency goes above 1kHz, representing a value of 0.25-0.42% at 10kHz of the modulation frequency. Curves A, and B in FIG. 4 show respectively the distortions when the ceramic filters are changed, from which it is understood that there are considerable fluctuations in characteristics.